Anyone else's ex stirring up trouble w/your kids?

I just found out last nite that my son is talking about moving in w/his dad when he turns 13. My husband took him shopping w/him, this is the first I have ever heard of it. I was so upset, I couldn't hardly sleep. I just always thought he would be w/me until he goes out on his own, ya know? His dad can't keep a legit job and I think he still does drugs, so this really scares me. I wish his dad wouldn't put these things in his head. I don't really think he could get hime if we go to court, but it really hurts my feelings that my son wants to leave me. He has a good life here, I just don't understand it. He would end up living w/his grandma, not his dad....

Honestly, we never really butted heads about anything up until now. I don't understand why he wants to start this after all this time. But, I did find out from a friend of mine who works for a lawyer that my son would have to be 16 to choose to go w/his dad. If he does before then, his dad would have to prove me unfit, and that's not gonna happen. My son assured me that he doesn't want to go anywhere, so I feel better now.
Thanks for responding and HAPPY HOLIDAYS!!